Heart valve surgery is used to repair or replace diseased heart valves. Valve surgery is an open-heart procedure conducted under general anesthesia. An incision is made through the patient's sternum (sternotomy), and the patient's heart is stopped while blood flow is rerouted through a heart-lung bypass machine.
Valve replacement may be indicated when there is a narrowing of the native heart valve, commonly referred to as stenosis, or when the native valve leaks or regurgitates. When replacing the valve, the native valve is excised and replaced with either a biologic or a mechanical valve. Mechanical valves require lifelong anticoagulant medication to prevent blood clot formation, and clicking of the valve often may be heard through the chest. Biologic tissue valves typically do not require such medication. Tissue valves may be obtained from cadavers or may be porcine or bovine, and are commonly attached to synthetic rings that are secured to the patient's heart.
Valve replacement surgery is a highly invasive operation with significant concomitant risk. Risks include bleeding, infection, stroke, heart attack, arrhythmia, renal failure, adverse reactions to the anesthesia medications, as well as sudden death. Two to five percent of patients die during surgery.
Post-surgery, patients temporarily may be confused due to emboli and other factors associated with the heart-lung machine. The first 2-3 days following surgery are spent in an intensive care unit where heart functions can be closely monitored. The average hospital stay is between 1 to 2 weeks, with several more weeks to months required for complete recovery.
In recent years, advancements in minimally invasive surgery and interventional cardiology have encouraged some investigators to pursue percutaneous replacement of the aortic heart valve. See, e.g., U.S. Pat. No. 6,168,614. In many of these procedures, the replacement valve is deployed across the native diseased valve to permanently hold the valve open, thereby alleviating a need to excise the native valve and to position the replacement valve in place of the native valve.
In the endovascular aortic valve replacement procedure, accurate placement of aortic valves relative to coronary ostia and the mitral valve is critical. Valve anchors comprising standard self-expanding stent systems are expected to have very poor accuracy in deployment, however. In a typical deployment procedure, the proximal end of the stent is not released from the delivery system until accurate placement is verified by fluoroscopy. The stent may jump to another position once released, making it impossible to know where the ends of the stent will be after release with respect to the native valve, the coronary ostia and the mitral valve.
Also, visualization of the way the new valve is functioning prior to final deployment is very desirable. Due to the expected jumping action of some self-expanding anchors, and because the replacement valve may not be fully functional before final deployment, visualization of valve function and position prior to final and irreversible deployment may not be possible with these systems.
Another expected drawback of prior art self-expanding replacement heart valve systems is their relative lack of radial strength. In order for self-expanding systems to be easily delivered through a delivery sheath, the metal needs to flex and bend inside the delivery catheter without being plastically deformed. Expandable stent designs suitable for endovascular delivery for other purposes may not have sufficient radial strength to serve as replacement heart valve anchors. For example, there are many commercial arterial stent systems that apply adequate radial force against the artery wall to treat atherosclerosis and that can collapse to a small enough of a diameter to fit inside a delivery catheter without plastically deforming However, when the stent has a valve fastened inside it, and that valve must reside within the heart, as is the case in aortic valve replacement, the anchoring of the stent to vessel walls takes significantly more radial force, especially during diastole. The force to hold back arterial pressure and prevent blood from going back inside the ventricle during diastole will be directly transferred to the stent/vessel wall interface. Therefore, the amount of radial force required to keep the self-expanding stent/valve in contact with the vessel wall and not sliding will be much higher than in stents that do not have valves inside of them. Moreover, a self-expanding stent without sufficient radial force will end up dilating and contracting with each heartbeat, thereby distorting the valve, affecting its function and possibly causing it to migrate and dislodge completely. Simply increasing strut thickness of the self-expanding stent is not a good solution as it increases profile and/or a risk of plastic deformation of the self-expanding stent.
In view of drawbacks associated with previously known techniques for endovascularly replacing a heart valve, it would be desirable to provide methods and apparatus that overcome those drawbacks.